The present invention relates to a data processing technology which realizes in a storage device a log data caching that is normally performed by a data processing device which executes transaction processing.
In recent years there are growing calls for computer systems to provide uninterrupted services and a system down time is considered as a loss. To reduce the system down time, a fast system recovery technology is essential and a recovery support system using a storage technology in particular is being called for.
In a conventional database management system, a host computer transfers data blocks and log blocks representing data block history to and from a storage device subsystem. To improve an efficiency of input/output processing, a buffer is set on a memory of the host computer and the data blocks and log blocks are cached on this buffer to minimize the input/output processing between the host computer and the storage device.
In the database management system, if in the event of a failure the recovery processing were executed using only logs, a recovery time would become inhibitively large. To minimize the recovery time, a checkpoint is performed periodically to guarantee a database consistency. When a checkpoint is retrieved, updated data blocks cached in the buffer are all written out onto the storage device to store management information required for the system restarting processing. Thus, since the checkpoint becomes a starting point at which the system restarting processing is initiated, the logs older than the checkpoint are not necessary, making the recovery time that much shorter. Generally, the checkpoint is often performed when the number of log blocks output during operation reaches a predetermined value. Such a technology is described in Abraham Silberschatz, Henry F. Korth, S. Sudarshan: “Database System Concepts”, McGraw-Hill Companies, 1997, pp. 511-535.
A technology for improving an efficiency of input/output processing on the storage device subsystem side uses a disk cache. If data requested by the host computer exists in the disk cache, the data can be retrieved simply by transferring the data in the disk cache to the host computer without accessing the disk device, thus speeding up the input/output processing. A conventional method of controlling an allocation of disk caches applies a LRU (Least Recently Used) algorithm and a FIFO (First-In First-Out) algorithm according to an operation mode specified by an operating system on the host computer. A means to improve a disk cache hit rate is described in WO99/40515. The proposed method to perform an efficient control of disk cache allocation in an environment where a plurality of applications are running multiplexingly, involves retrieving an access frequency for each divided area on the disk device and, when allocating disk caches, flushing the data blocks stored in the disk caches beginning with the data block whose access frequency is low.